U.S. Pat. No. 4,732,525 (corresponding to SE 452279) teaches a parallel-kinematical machine in the form of a robot of conventional design. The robot includes three setting devices, which can be lengthened and shortened, in combination with a central tube that carries a positioning head at one end thereof. The central tube is also mounted for movement in its axial direction through the medium of a central bearing in the form of a universal joint, which provides three degrees of freedom in relation to the machine base. Each setting device is connected to the positioning head via a joint that provides three degrees of freedom and also to the machine base via a joint that has two degrees of freedom, so as to enable the positioning head to move within a limited working range. The setting devices take-up solely tensile forces and pressure forces, whereas the central tube takes-up all rotational forces and bending stresses from loads on the positioning head.
The accuracy of the movements of such a machine depends greatly on its rigidity, which, in turn, depends on the number of bearings/degrees of freedom available and also on the ability of the component materials to minimise torsional stresses and bending stresses in critical directions. For instance, it can be mentioned that large lateral forces in respect of the positioning head result in a tendency of the central tube to bend and/or to be rotated between its gyro bearing in the machine base and its connection with the setting devices.
The rigidity of the described conventional machine thus depends, among other things, on the design of the connection of the setting devices with the positioning head and also on the intrinsic rigidity of the central tube per se. In order to enhance the rigidity of such a conventional machine it is necessary, primarily, to apply stricter tolerances in each joint and, secondarily, to use a more robust central tube, therewith adding a weight increase.
The central tube can be made more robust, by using a stiffer material and/or by increasing the thickness of the tube and/or increasing its diameter.
All such improvements in machine rigidity, however, result in higher costs, heavier machine and a reduction in the working area within which the positioning head can be manoeuvred.
Similar parallel-kinematical machines are also known, for instance, from UK Patent Application 8319708 (2,143,498), U.S. Pat. No. 4,569,627 and NO 148216.
However, none of these known machines has a basic construction that permits the level of rigidity and therewith the level of accuracy to which modern machines aim.